Sticking with lower case can remove potential ambiguities on capitalisation and reduce the chance of errors. Many CMSs follow that policy and automatically strip special characters, replace spaces with dashes and convert to lower case.

I used to mix case in my directory/filenames and eventually got tired of dealing with all the 404 errors that other people's mistyped links created.

If you have a high percentage of 404 errors for UPPER CASE or MiXeD CaSe directory/filename URLs then in my opinion it would be worthwhile to switch everything to lower case, implement redirects, and just use lower case going forward.

Don't do this out of paranoia -- only do it if the people who link to your site or who type your URLs into their browsers are hitting 404 pages.